Abstract

Background

Patterns of biodiversity in the subterranean realm are typically different from those
encountered on the Earth’s surface. The Dinaric karst of Croatia, Slovenia and Bosnia
and Herzegovina is a global hotspot of subterranean biodiversity. How this was achieved
and why this is so remain largely unresolved despite a long tradition of research.
To obtain insights into the colonisation of the Dinaric Karst and the effects of the
subterranean realm on its inhabitants, we studied the tertiary relict Congeria, a unique cave-dwelling bivalve (Dreissenidae), using a combination of biogeographical,
molecular, morphological, and paleontological information.

Results

Phylogenetic and molecular clock analyses using both nuclear and mitochondrial markers
have shown that the surviving Congeria lineage has actually split into three distinct species, i.e., C. kusceri, C. jalzici sp. nov. and C. mulaomerovici sp. nov., by vicariant processes in the late Miocene and Pliocene. Despite millions
of years of independent evolution, analyses have demonstrated a great deal of shell
similarity between modern Congeria species, although slight differences in hinge plate structure have enabled the description
of the two new species. Ancestral plesiomorphic shell forms seem to have been conserved
during the processes of cave colonisation and subsequent lineage isolation. In contrast,
shell morphology is divergent within one of the lineages, probably due to microhabitat
differences.

Conclusions

Following the turbulent evolution of the Dreissenidae during the Tertiary and major
radiations in Lake Pannon, species of Congeria went extinct. One lineage survived, however, by adopting a unique life history strategy
that suited it to the underground environment. In light of our new data, an alternative
scenario for its colonisation of the karst is proposed. The extant Congeria comprises three sister species that, to date, have only been found to live in 15
caves in the Dinaric karst. Inter-specific morphological stasis and intra-specific
ecophenotypic plasticity of the congerid shell demonstrate the contrasting ways in
which evolution in the underground environments shapes its inhabitants.